In a reactor core, which is a part that reacts fuel in a reactor, a plurality of fuel rods are arranged therein in a state of a fuel assembly having a bundle of fuel rods, and the circumference of the fuel assembly is filled with light water used as a primary coolant or a moderating material. In a pressurized water reactor (PWR), which is one type of reactors, a path at the time of extracting energy is separated into a primary cooling system and a secondary cooling system. In the primary cooling system, light water exposed to heat at the time of a fuel reaction is made high-temperature and high-pressure water by pressurizing light water circulating in the reactor including the reactor core so that light water does not boil. In the secondary cooling system, light water circulating in the secondary cooling system is exposed to heat of the high-temperature and high-pressure water in the primary cooling system so that light water is brought to a boil, and energy is extracted as high-temperature and high-pressure steam.
In the pressurized water reactor, a plurality of fuel assemblies are mounted on a lower reactor-core plate provided in a lower part of a reactor vessel, in a state with the fuel assembly being mounted on a lower nozzle. A plurality of holes are formed on the lower reactor-core plate and the lower nozzle, and light water circulating in the reactor core flows upward from below the reactor-core plate, passes through the holes in the lower reactor-core plate and then the holes in the lower nozzle, and flows toward the fuel assembly on the lower nozzle. Accordingly, the light water circulates in the primary cooling system, while being exposed to the heat at the time of a fuel reaction.
In the pressurized water reactor, at the time of circulation of light water, light water passes through the holes in the lower reactor-core plate and in the lower nozzle and flows to the circumference of the fuel assembly. However, a flow rate of light water flowing to the circumference of the fuel assembly may be different according to an arrangement position of the fuel assembly. Further, when the performance of the reactor at the time of operation is considered, it may be desired to adjust the flow rate thereof to the fuel assembly. Therefore, the conventional reactor may have a structure for adjusting the flow rate of light water flowing to the fuel assemblies.
For example, in a reactor described in Patent Literature 1, a flow-resistance changing member is provided on a lower reactor-core plate so that the flow resistance of holes formed in a central part of the lower reactor-core plate is increased with respect to the flow resistance of the holes formed on a periphery of the lower reactor-core plate, of the holes formed on the lower reactor-core plate. When light water passes through the holes in the lower reactor-core plate, such flow distribution is likely to occur that the flow rate increases in a central part than in the periphery of the lower reactor-core plate due to an influence of structures in the reactor. By providing a flow-resistance changing member to change the flow resistance in this manner, the flow distribution can be equalized.
In a fuel assembly for a pressure-loss variable pressurized water reactor described in Patent Literature 2, a pressure-loss adjusting plate having a pressure-loss adjusting element fixed thereon, which is arranged at a position corresponding to holes formed in a lower nozzle, is fixed by a screw on a lower surface of the lower nozzle. Accordingly, the flow rate of light water to the fuel assembly positioned on the lower nozzle can be adjusted for each of fuel assemblies.